The elusive agent of electroweak symmetry breaking

- an experimentalist point of view -

Ulrich Heintz

Brown University

the holy grail of particle physics

3/11/2008 Ulrich Heintz - Sakurai Prize Celebration

the quest for the Higgs boson at

LEP

Tevatron

LHC

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Gerry’s particle

H searches before LEP• experiments at PSI, SPS, CESR

• excluded mH < 5 GeV– subject to significant uncertainties

3/11/2008 Ulrich Heintz - Sakurai Prize Celebration

CUSB, PRD 35, 2883 (1987)X+exclude 2m < mH < 5 GeV

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LEP

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e+e collisions 1989-1995: Ecom = MZ

1996-2000: Ecom 209 GeV

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LEP 2LEP 1

H production at LEP

Center of mass energy (GeV)

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Björken process Higgsstrahlung

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Very little background expected

Events expected at LEP

among 2x107 Z

H search at LEP1

for mH<2mW : H bb (~80%)

0.0 mH 65 GeV/c2

Excluded at 95% C.L.

for mH<2m : H ee,

for mH<2mb: H gg, cc,

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H searches at LEP2• Decay signature defined by Z decay

Higgs Z Fraction

bb qq 51.5% bb 14.7% Any ll 6.7%

bb 2.5% qq 5.0% Total 80.9%

“4-jets”“missing E” “leptons”

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and then…

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discovery or fluctuation?• 3 selections with increasing purity for mH = 115 GeV

“the results of the experiment were inconclusive so we had to use statistics...”from Louis Lyon’s book “statistics for nuclear and particle physicists”

Data 5Bkg 3.9Sig 3.8

Data 17Bkg 15.8Sig 7.1

Data 4Bkg 1.2Sig 2.2

S/B for mH > 109 GeV/c2

> 0.5 >1 >2

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the final LEP limit

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modified frequentist confidence levels

)( obsb LLPCL

define likelihood ratio

)|(

)|(

bdatap

bsdatapL

)( obsbs LLPCL

for background

signal + background

b

bss CL

CLCL

Signal can be excluded at 95% CL if CLs < 0.05

how much less likely is observed data for s+b

than for b only?

mH > 114 GeV

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the end of LEP

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2 km

Tevatron

pp collisions Ecom = 2 TeVL = 41032/cm2/s (2 fb-1/year)analyzed now: Ldt = 5 fb-1

expect Ldt = 10 fb-1 by end of 2011

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H at the Tevatron?

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H production at Tevatron

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gluon fusion439 fb

associate productionWH: 51 fbZH: 33 fb

vector boson fusion39 fb

@MH=160 GeV

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H search at Tevatron

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mH < 135 GeV (Hbb) background drivenWHlbb, ZHll/bb

mH > 135 GeV (HWW) rate drivenall production modes

“low mass” Higgs “high mass” Higgs

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H search at Tevatron• example channel: WHlbb

+ 2 b-tags

e/ + pT + 2-3 jetsfor mH = 115 GeVexpect 24 Higgs decays in 84000 events

expect 7 Higgs decaysin 700 events

use several variables to form neural network discriminant

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H search at Tevatron

neural network

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single top production• same final state as low mass Higgs• slightly larger cross section

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Tevatron H limits

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published inPRL 104, 061802 (2010)by CDF and D0 Collaborations

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sensitivity of H search at Tevatron• assume analysis will stay the same• known improvements will be implemented

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146-183

136-190

“low mass” Higgs “high mass” Higgs

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LHC pp collisions end 2011: Ecom = 7 TeV, Ldt = 1 fb-1

end 2014: Ecom = 14 TeV, Ldt = 25 fb-1

end 2016: Ecom = 14 TeV, Ldt = 100 fb-1

Peter Jenni (Moriond 2010)

CMS

Alice

ATLAS

LHCb

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H production at LHC

Higgs production cross sections at LHC

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CMS projections for H search

• mH < 130 GeV– H

• 130 < mH < 190 GeV– H W+W- ℓℓ

• 190 < mH < 460 GeV– H ZZ ℓℓ ℓℓ

• 460 < mH < 800 GeV– H W+W- ℓqq

designed to discover Higgs with 100 < mH < 800 GeV

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hard to see: H bb

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the CMS experiment• homogeneous electromagnetic calorimeter

– 78,000 lead tungstate crystals– excellent photon energy resolution– measure H

M (GeV)

mH = 130 GeVL = 100 fb-1

# of

eve

nts

/ 0.

5 G

eV

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LHC projection for next 2 years

exclude 145 < mH < 190

GeV

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LHC projection for next 2 years

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summary

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• the search for Gerry’s boson has shaped particle physics experiments for the last three decades

• but it still eludes detection• the quest goes on…

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Congratulations, Gerry!

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